The present invention relates to a process for carrying out an antireflection treatment on a substrate made of an organic material, in particular an ophthalmic lens, using a technique of deposition by vacuum evaporation. Such coatings are generally produced by means of metal oxides having high and low refractive indices.
The effectiveness of an antireflection treatment depends to a great extent on the value of the refractive indices of the layers deposited. The constraints associated with the type of deposition used and the nature of the substrates to be treated restrict the materials that can be used for these treatments.
Manufacturers are being required to devise antireflection treatments of ever higher performance. For information, in the ophthalmic field, the effectiveness of such a treatment is given in terms of reflection per face, which lies between 1.6 and 2.5% for a poorly effective treatment, is between 1.0 and 1.8% for a moderately effective treatment and must nowadays be between 0.3 and 0.8% for a highly effective treatment.
These constraints mean that novel materials have to be sought, these sometimes being difficult to develop from an industrial standpoint. Another approach may consist in using materials previously reserved for substrates made of inorganic material.
To produce a highly effective antireflection treatment, it is necessary for the last layer deposited to have the lowest possible refractive index. In the case of organic substrates, SiO2 is often used because of its refractive index of 1.48, its good adhesion, scratch resistance and corrosion resistance properties and its ease of deposition.
MgF2 is very widely used for substrates made of an inorganic material because of its very low refractive index, namely 1.38. A drawback of this material lies in its friability and its lack of adhesion when it is deposited at temperatures below 200° C.
In general, MgF2 is not used for organic substrates as these cannot be heated above 150° C. as they undergo yellowing and deterioration.
It is therefore necessary to find a process allowing deposition of MgF2 on an organic substrate (“cold” deposition) and to find other materials which might be suitable.
JP 8-236635 teaches the deposition of an MgF2 layer on an organic substrate by means of the technology of sputtering. However, the layers deposited by sputtering have particular physico-chemical characteristics. In particular, the layers deposited are generally denser, which poses an adhesion problem. The document teaches that such deposition carried out by means of vacuum evaporation results in layers having a low degree of crystallization, which entails an insufficient abrasion resistance.
A multilayer antireflection coating on an organic material in the ophthalmic field, the final layer of which is made of MgF2, is also disclosed in the document EP-A-619 504. That document indicates that the layers disclosed are produced by plasma-enhanced processes and within certain unspecified limits by sputtering. The most economic and most widely used deposition process is however non-plasma-enhanced vacuum evaporation. Layers obtained by plasma-enhanced evaporation furthermore have a higher density than those obtained without enhancement, but high density means that these layers are highly strained, which may result in adhesion of inferior quality.
Document JP 61250601 also discloses a multilayer antireflection coating on an organic substrate. According to that document, SiO2 is used as the outer layer of low refractive index and preferably a multilayer coating consisting of three layers of the Y2O3/TiO2/SiO2 type is used. During deposition of the multilayer coating, the interface of at least one of the layers is treated by ion bombardment. This treatment improves the adhesion of the layers. The use of MgF2 as material having a low refractive index is neither disclosed nor suggested in that document.
Document JP 7076048 discloses a multilayer antireflection coating deposited on an organic substrate. This specifies that the outer layer, made of MgF2, is deposited at low temperature with ion assistance using the IAD (Ion Assisted Deposition) technique and that surface on which this layer is deposited is subjected beforehand to a cleaning treatment by ion bombardment and/or by plasma. However, it turns out that the use of the IAD technique has a number of drawbacks. This type of assistance is generally used to densify the layers deposited. But on the downside, the densest layers have lower adhesion. Furthermore, ion assistance may make the MgF2 layer more absorbent, this being a troublesome drawback in the case of ophthalmic applications. Finally, this type of assistance makes the process more complicated and increases the production costs substantially.